In a typical cellular radio system, also referred to as a wireless communication system, user equipments, also known as mobile terminals and/or wireless terminals communicate via a Radio Access Network (RAN) to one or more core networks. The user equipments may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile terminals, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node referred to as a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “eNB”, “eNodeB”, “NodeB” or “B node”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a radio network node referred to as a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using WCDMA for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. In the end of 2008 the first release, Release 8, of the 3GPP Long Term Evolution (LTE) standard was finalized and the release 9 is currently going on.
Within the 3GPP specifications for LTE, the Evolved UMTS Terrestrial Radio Access (E-UTRA) describes the Radio Access Technology. Mobile part of LTE is referred to as User Equipment (UE), and the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) describes the radio access network, referring to the base station part containing the Evolved Node B (eNB) and which is a radio network node. Along with the LTE specifications, 3GPP is working on a complementary project called the System Architecture Evolution (SAE), which defines the split between LTE and a new Evolved Packet Core (EPC). This architecture is a flatter, packet-only core network that will help deliver the higher throughput, lower cost, and lower latency that is the goal of LTE. It is also designed to provide seamless interworking with existing 3GPP and non-3GPP access technologies.
If a Mobility Management Entity (MME) needs to signal with the user equipment that it is in Evolved Packet System (EPS) Connection Management (ECM)-IDLE state, e.g. to perform an MME/Home Subscriber Service (HSS)-initiated detach procedure for the ECM-IDLE mode user equipment, or a Serving GateWay (SGW) receives control signalling, e.g. Create Dedicated Bearer Request or Modify Dedicated Bearer Request, the MME starts network triggered service request procedure.
If Idle mode Signalling Reduction (ISR) is activated, when the SGW receives a Create Dedicated Bearer Request or Modify Bearer Request for a user equipment, and the SGW does not have a downlink S1-U and the Serving General Packet Radio Service (GPRS) Support Node (SGSN) has notified the SGW that the user equipment has moved to Packet mode Mobility Management (PMM)-IDLE or STANDBY state, the SGW buffers signalling messages and triggers MME and SGSN to page the user equipment. S1-U is the communication interface between the MME and SGW. In this case the SGW will be notified about the current Radio Access Technology (RAT) type based on the user equipment triggered service request procedure. The SGW will go on executing the dedicated bearer activation or dedicated bearer modification procedure, i.e. send the corresponding buffered signalling to MME or SGSN which user equipment resides in now and inform the current RAT type to a Packet Data Network Gateway (PGW) if the RAT type has been changed compared to the last reported RAT Type. If dynamic Policy and Charging Control (PCC) is deployed, the current RAT type information shall also be conveyed from the PGW to a Policy and Charging Rules Function (PCRF). If the PCRF response leads to an EPS bearer modification the PGW should initiate a bearer update procedure.
Paging in an LTE network is the process of notifying user equipments in idle mode i.e. so-called RRC_IDLE mode, according to the Radio Resource Control (RRC) protocol, about an incoming data session. In the LTE network, the location of a user equipment in idle mode is known by the network on a Tracking Area (TA) granularity.
The Mobility Management Entity (MME) in the EPC initiates the paging procedure by sending a Paging message to all the eNBs with at least a cell belonging to one or several TAs, in a so called Tracking Area List within which the user equipment is registered. So for each incoming “call” towards an Idle user equipment, pagings will be sent out in ALL cells belonging to the TAs in the in the TA list
If a user equipment reselects a cell belonging to a new TA where the user equipment is not registered, the user equipment initiates a TA update to the EPC in order to register and it receives a new list of TAs where it is registered.
The Air Interface
Radio resources in E-UTRA are in the time domain defined by Radio Frames and Subframes. Each Radio Frame is 10 ms and consists of 10 Subframes each with a duration of 1 ms.
The user equipment may use Discontinuous Reception (DRX) in idle mode. DRX is an essential feature in cellular systems to allow user equipment battery saving. When the user equipment is in its recurring so called DRX periods it is not listening to the eNB and when it is in its non-DRX periods it is “awake” or listening to the eNB.
In the LTE idle state the user equipment listens to the network (e.g. by means of paging information) only at non-DRX instants and performs autonomous cell reselection. In order to receive data the user equipment needs to enter into LTE active state.
Hence paging a user equipment must occur during user equipment's non-DRX periods. The broadcast of Paging messages by eNB is done only in certain sub-frames, so called Paging Occasions (PO), of certain Radio Frames, so called Paging Frames (PFs).
According to 3GPP, Configuration of PFs and POs is done in the eNB by the 2 parameters defaultPagingCycle and nB. These two parameters are used by eNB and the user equipment in calculations of the occurrences of POs in the time domain.
Where the default paging cycle is the length of the cycle for PO recurrence for user equipments in time. I.e. the defaultPagingCycle=320 ms means that each user equipment should wake up from its DRX and listen for Paging each 320 ms. nB affects number of available POs per PF. Both parameters affect position of the PFs and POs in time domain used by the user equipment.
The defaultPagingCycle and nB are broadcasted in System Information, so that the user equipments adapt their DRX behavior according to eNB configuration. System Information is broadcast by eNBs in all cells and contains necessary information about the Radio Network's properties and configuration in order for the user equipment to be able to connect and communicate with the radio access network.
Upon reception of paging the user equipment initiates a Service Request to the network, which is expected by the MME in EPC. This is done in order to inform the network about where the user equipment is located and to start up the establishment of a connection for receiving the incoming data.
Paging traffic is for many operators a concern and subject to “tuning” and “optimization” in terms of Tracking Area planning. A high intensity in Paging load always means lower User Data rates in LTE, since paging traffic and data traffic share the same physical radio resources. Thus today's inefficient paging systems reduces the performance of the network.